The present invention relates generally to target detecting radar systems and more particularly to radar systems for detecting targets having a radial velocity relative to the system.
A radar system, in general, includes a transmitter adapted to radiate short duration, high frequency, pulses of electromagnetic energy and a receiver responsive to the echo pulses returned when the radiated pulses impinge upon an object. The information provided by the echo pulses is derived and presented on an indicating device.
A radar system which is adapted to distinguish between fixed objects and moving objects and which applies to the indicating device signals representing only moving targets is referred to as a moving target indicating (MTI) system. Distinction between the moving and the fixed targets is normally attained by utilizing the Doppler effect i.e., the phase change of the echo with respect to the transmitted pulse caused by the radial velocity of the target with respect to the radar system. In contrast, the phase relationship between a transmitted pulse and an echo resulting from a fixed target is the same for successive pulses. The fact that for successive pulses the relative phase of the transmitted pulse and the echo varies when the target is moving, and is constant when the target is fixed, provides a means for distinguishing between fixed and moving targets.
Many prior art MTI systems utilize a two pulse technique to detect moving targets. This technique comprises the transmission of a first pulse followed by a second pulse, and then the subtraction of the echo returns from these two pulses. This subtraction step removes the echoes from fixed targets and slowly moving targets referred to as clutter. A major problem with this technique for clutter cancellation is that "blind-speeds" exist at which no MTI output is produced. "Blind-speeds" occur when the target moves toward or away from the radar system a distance equal to an integral number of half wavelengths of the transmitter carrier frequency during the interval between pulses. A further problem arises due to range ambiguities of the system. This "blind-speed" problem is solved by utilizing oppositely chirped pulses for the two pulse system. This type of design takes advantage of the opposite range-doppler-coupling on up frequency sweeps and down frequency sweep transmissions. When the echos from these oppositely swept pulses are subtracted, the echoes from non-moving targets again appear at the same time after transmission on both pulses and cancel. However, due to the opposite range-doppler-coupling on the two different sweeps, echoes from moving targets couple in opposite directions and thus do not cancel, even if the target velocity is such as to cause a blind-speed.
One of the problems with this type of two-pulse system is that the standard antenna pattern has a sin X/X pattern for scanning. This pattern is shown in FIG. 1 by the curve 10. Assume that each vertical line 12 in the antenna scan pattern 10 represents an expanded chirp pulse. It can be seen that as this antenna pattern 10 is scanned in the direction 14 through a target 16, the echoes from the individual expanded chirp pulses 12 reflected from the target 16 will have different gains. This type of gain modulation caused by the shape of the antenna pattern and the antenna scanning motion is referred to as antenna scan modulation.
From the above, it can be seen that no two successive pulses will have the same amplitude. Thus, successive pulses 12 and 12A reflected from a target 16 which is stationary will come back with the same phase, but will have different amplitudes. Thus, only a very poor clutter cancellation is realized.
This antenna scan modulation caused by different gains as the antenna passes by the fixed target can be obviated to some extent by significantly increasing the number of pulses per scan. Then, each pulse will be very close to its adjacent pulse and will have a very similar gain magnitude to its adjacent pulse. However, there is a certain minimum desired interpulse period required in order to prevent long range echoes from the first pulse in the sequence from arriving at the radar receiver at the same time as short-range echoes from the second pulse. In order to prevent this mixing of echoes from short range clutter on one pulse with long range clutter from previous transmitted pulses the interpulse period is required to be long compared to the time it takes for the radar pulse to travel out to the clutter and back.
It can be seen that with this interpulse period limitation and a large number of transmitted pulses, the antenna scan rate must be very slow. However, there is also a requirement to update the target detection information once approximately every four seconds in order to maintain track.
Thus it can be seen that the radar pulses or echo hits must be widely spaced on the antenna pattern 10. There must be some means for compensating for this difference in gain from pulse-to-pulse in order to cancel clutter. One very complicated prior art system for accomplishing this clutter cancellation is disclosed in U.S. Pat. No. 3,225,349 by Thor. The Thor system discloses the use of four or a higher even number of chirped pulses in FIG. 7 of the reference. One set of adjacent pulses is chirped with a slope of T while a second set of adjacent pulses is chirped with a slope of 2T. Thor then discloses a complicated processing system for effecting this cancellation.
At the present time there is no known method or apparatus for a three pulse chirp MTI system. However, such a three pulse system is clearly the most economical system for compensating for antenna scan modulation.